A positive electrode active material for a lithium ion secondary cell, in which the amount of a transition metal present in the vicinity of the outermost surface thereof is significantly decreased is provided. A solid electrolyte-coated positive electrode active material powder contains particles of a positive electrode active material for lithium ion secondary cell, containing a composite oxide of Li and a transition metal M, having on a surface thereof a coating layer of a solid electrolyte represented by Li1+XAlXTi2−X(PO4)3, wherein 0 £ X £ 0.5. An average proportion of a total atom number of Al, Ti and P in a total atom number of Al, Ti, M and P within an etching depth of 1 nm from the outermost surface determined by analysis in a depth direction with XPS is 50% or more. The transition metal M is, for example, at least one kind of Co, Ni and Mn.